1. Field of the Invention
The present invention relates to tubes with concentric, cylindrical power electrodes. These tubes are, for example, triodes or tetrodes.
A triode tube comprises mainly a central cylindrical cathode emitting electrons when it reaches a sufficient temperature, with a control grid around the cathode and an anode surrounding the control grid. The electrons emitted by the cathode go through the grid and reach the cathode, if the potential of the grid and of the anode have appropriate values. Tetrodes have an additional grid, called a screen grid, inserted between the control grid and the anode.
2. Description of the Prior Art
The cathode is often made out of two sheets of emissive metal wires that are intersected to obtain a meshing. The assembly thus made has a cylindrical structure. Each end of the cylinder is fixed to a support. These cathodes are said to be caged.
The grids are also meshed. They may be made out of sheets of wires of a refractory material that are intersected to obtain a meshing. The wires are soldered to one another at each intersection. The assembly thus formed has a cylindrical shape, and its ends are connected to supports.
A second way of making a grid is to take a cylindrical sheet of refractory material and to pierce it with apertures that are regularly spaced out to obtain the meshing.
The material commonly used as a refractory material is pyrolitic graphite or molybdenum. Each mesh is defined by a succession of rods connected by their ends and the intersection between two rods is a node.
Because of this highly cut-out structure, the cathode and the grids are subject to vibrations that affect their mechanical stability. The distance between the cathode and the control grid is small. It is generally smaller than 1000 micrometers, and the vibrations that may occur cause appreciable variations in distance. These vibrations are detrimental to the efficient working of the tube. The same observations apply to the intergrid distances in the case of tetrodes or other multiple-grid tubes.
A measure of the importance of mechanical stability can be had if we add that the cathode may work at a high operating temperature (of the order of 1700.degree. C.) and that it should also have high resistance to deformation. The grids will attain a lower temperature (of the order of 1200.degree. C.) but should also stand up well to deformation.
Another condition that has to be integrated, in order to obtain efficient operation of the tube, is the tranparency of the grids. The rods and the nodes of the meshes form a barrier to the electrons coming from the cathode. The interception of a large number of electrons by a grid gives rise to a high grid current, especially in high-power tubes. This grid current prompts an additional heating of the grid and necessitates the use of a relatively powerful grid supply. The transparency of the grid depends on its geometry.
To make the grid, it is also necessary to take account of the distribution of the grid potential, between the rods. The potential must be distributed as regularly as possible. This is important for the control grid which is used to regulate the potential around the cathode. The latter condition also depends on the geometry of the grid.
An ideal grid, from the standpoint of potential and transparency, would have an infinity of very thin, vertical wires. The grid current would be very low, and the grid potential would be distributed very regularly around the cathode.
By contrast, this grid would have relatively poor mechanical resistance, especially if it were large sized.
This point has therefore led to the intersecting of the wires to increase the rigidity of the grid.
The grids that are frequently used have quadrilateral meshes, i.e. square, rectangular, rhombus-shaped or parallelogram-shaped meshes. Four rods leave one mesh node.
In high-power tubes, a grid of this type gets deformed, and it has been necessary to strengthen it by adding on rods: triangular meshes have been made. There are now six rods that leave each mesh node. The surface area of the nodes is greater, and so is the grid current.
The present invention seeks to overcome these drawbacks and proposes a gridded tube working with a lower grid current. To this end, it is sought to minimize the electron-interception surface area, in harming neither mechanical stability nor the distribution of the grid potential around the cathode.